The present invention relates to a press felt for papermaking (which is also called press fabric and hereinafter may be referred to simply as “felt”) used in papermaking machines.
In a papermaking process, to remove water from a wet paper web, conventional papermaking machines generally include a wire part, a press part, and a dryer part. The wire part, the press part, and the dryer part are arranged in this order in a wet paper web-transferring direction. The wet paper web is dewatered while being transferred to be sequentially passed on to papermaking apparatuses respectively arranged in the wire part, the press part, and the dryer part, and finally dried in the dryer part.
Each of the parts employs a papermaking apparatus corresponding to its dewatering function. A press apparatus arranged in the press part includes a plurality of press units disposed in series in the wet paper web-transferring direction.
Each of the press units includes an endless felt or a felt in which the ends of an open-ended felt are connected in a papermaking machine to be made into an endless shape, and as a press, a pair of rolls (namely, a roll press) or a roll and a shoe-housing cylindrical belt (namely, a shoe press), which are arranged in opposition at upper and lower positions in such a manner as to sandwich a part of each felt therebetween. A wet paper web transferred to the press unit by the felt running at approximately the same speed in the same direction is subjected to application of pressure by, together with the felt, the rolls or the roll and the shoe-housing cylindrical belt, whereby water is removed from the wet paper web to be successively absorbed by the felt.
Among such papermaking machines, there are, first, a papermaking machine with a roll press mechanism in which a press part includes a press unit for applying pressure to parts of felts holding a wet paper web therebetween by sandwiching the parts thereof between rolls, and, second, a papermaking machine with a shoe press mechanism in which a press part includes a press unit for applying pressure to parts of felts holding a wet paper web therebetween by sandwiching the parts thereof between a roll and a shoe-housing cylindrical belt.
The felt is composed of a base fabric and a batt fiber layer. The batt layer is arranged on both of a wet paper web-side surface and a roll-side surface of the base fabric or only on the wet paper web-side surface thereof. The batt fiber layer is formed by integrally intertwining batt fibers into the base fabric by needle punching. The basic function of the felt is to play the roles of squeezing water out of a wet paper web (water removal), increasing smoothness of the wet paper web, and transferring the wet paper web.
Among the functions of the felt, dewatering the wet paper web, in particular, is given great importance. By passing the wet paper web between a pair of rolls or a roll and shoe mechanism that apply pressure, water is moved from the wet paper web to the felt and the water in the felt is discharged outside the felt system. Therefore, emphasis is placed on maintaining water permeability and compressibility by appropriately securing a spatial volume in the felt.
To appropriately secure the spatial volume in the felt to maintain water permeability and compressibility, endless felts and open-ended felts have been developed in which warp and weft yarns forming base fabrics of the felts are monofilament yarns.
An open-ended felt is a so-called seamed felt. After installing the open-ended felt into a felt run of a papermaking machine, both ends in the length direction (both ends formed in such a manner so as to traverse a felt-running direction) of the felt are connected with each other to form an endless shape, thereby improving workability in felt engagement in the papermaking machine. In the seamed felt, warp yarns forming a base fabric are folded back to form seam loops at both ends in the length direction of the felt to be alternately intermeshed such that respective center holes of the loops are aligned, whereby there is formed a common hole, through which a core line is inserted to connect both ends of the felt.
The warp yarns forming the base fabric of the seamed felt are monofilament yarns so that an appropriate spatial volume can be secured in the felt to maintain water permeability and compressibility as described above. On the other hand, from a different viewpoint, when considering objectives such as retaining a loop shape, intermeshing the loops, and inserting the core line, using monofilament yarns is inevitable.
However, when monofilament yarns are used as warp and weft yarns forming the base fabric of a felt, the task of intertwining of monofilament yarns forming the base fabric and the batt fibers by needle punching is delicate. As a result, shedding of batt fibers occurs during use of the felt, a phenomenon called dehairing.
The dehairing phenomenon causes various problems. For example, transferability of a wet paper web is disturbed and the shed batt fibers adhere to the wet paper web, causing a paper printing problem. In addition, due to the shedding of the batt fibers, physical properties of the felt, such as water removability and water permeability, are changed to destabilize the operation of papermaking, and also, warp and weft yarns of the wet paper web-side surface or the roll-side surface of the base fabric are exposed, which accelerates the abrasion of the base fabric to reduce the strength thereof, resulting in shortening of the felt life.
In addition, since monofilament yarn has excellent flattening-resistant characteristics, it appropriately secures a spatial volume in felt. Meanwhile, intersections between the warp yarns and the weft yarns of a base fabric, that is, knuckle portions are emphatic. Thus, another problem occurs in which during dewatering, the emphatic knuckle portions are transcribed to the wet paper web to cause reduction in surface smoothness of the wet paper web (paper mark).
Furthermore, by using monofilament yarns as warp and weft yarns forming the base fabric of the felt, the yarns slip against each other because of smoothed surfaces of the monofilament yarns, whereby the warp yarns project from the felt surface or shed during the use of the press felt.
To prevent dehairing of batt fibers of a felt and improve the smoothness of a wet paper web, various felts have been proposed.
JP-A-2009-68153 proposes a felt using a base fabric made of multifilament yarns each formed by bundling a plurality of pieces of filaments having a fineness of 100 dtex or lower as at least one of a warp yarn and a weft yarn arranged on a wet paper web-side of the base fabric.
As an example of the felt described in JP-A-2009-68153, a felt has been described including a laminate structure formed by overlapping two base fabrics: a wet paper web-side base fabric and a roll-side base fabric. Usually, in the laminate structure, the two base fabrics are woven, subjected to thermal setting to adjust to an appropriate size difference, and then bonded together. Upon bonding together, the base fabrics may not be able to be bonded to each other due to a size difference between them. In this case, thermal setting is needed again, so that employing the laminate structure is very costly.
In addition, employing multifilament yarns as the warp or weft yarns of the base fabric arranged on the wet paper web-side is not very effective in the prevention of shedding of batt fibers on the roll-side surface, although it can prevent shedding of batt fibers on the wet paper web-side surface.
JP-A-2010-196206 proposes a felt in which a base body is made of a plurality of reinforcing materials, and the reinforcing material on a wet paper web-side surface adjacent to a wet paper web-side batt layer or the reinforcing material on a roll-side surface adjacent to a roll-side batt layer includes a spun yarn that is integrated into the base fabric to such an extent that yarn configuration for intertwining the spun yarn with each batt layer disappears.
The base fabric of the felt described in JP-A-2010-196206 also has a laminate structure. Thus, production cost is very high, as in the felt described in JP-A-2009-68153. Additionally, due to the incorporation of a spun yarn whose yarn configuration disappears into the base fabric, there is a possibility that compressibility of the base fabric is inhibited.
JP-A-2010-100947 proposes a felt in which a base fabric has a multilayer structure obtained by laminating layers of a plurality of ground warp yarns formed of monofilament yarns such that the yarns overlap each other in a thickness direction of the cloth, and an additional warp yarn made of a yarn material different from that of the ground warp yarns forms only the layer(s) of the ground warp yarns on a papermaking face-side and/or a running face-side to be interwoven in such a manner as if floating and sinking on a surface of the base fabric on the papermaking face-side or the running face-side.
However, in the felt described in JP-A-2010-100947, the additional warp yarn arranged only on the papermaking face-side is likely to cause fiber shedding on the running face-side. Conversely, when the additional warp yarn is arranged only on the running face-side, fiber shedding is likely to occur on the papermaking face-side.
Furthermore, in a seamed felt in which an additional warp yarn is arranged on both of the papermaking face-side and the running face-side, as disclosed in JPA-2010-100947, upon core-line replacement (such as the replacement of a core line damaged by needling when batt fibers are integrally intertwined into a base fabric by the needling) and felt splitting at a seam portion (such as the cutting of a bottom batt fiber layer of the seam portion, the removal of the batt fibers entangled with seam loops, and the cutting of a top batt fiber layer of a seam portion, which are a so-called flap processing), the additional warp yarn is connected to upper and lower parts of the seam portion. Thereby, the seam loops cannot be seen due to the presence of the additional warp yarn, and the seam portion cannot be opened, which significantly increases working steps. In addition, when cutting the additional warp yarn, there was a possibility of damage to the seam loops, resulting in cutting down of them.